Add espressif directory to .mbedignore

The old mbed CLI 1 tool, currently used to compile the MCUboot port for
Mbed™ OS, compiles all files unless otherwise specified in .mbedignore.
The espressif directory in the boot folder was added to the ignore file
to prevent failed builds.

Signed-off-by: Sudarshan Sreeram <sudarshan.sreeram@arm.com>
Signed-off-by: David Brown <david.brown@linaro.org>